The present invention relates to the provision of timing reference signals to one or more radio heads.
Many wireless communication systems include one or more radio heads connected to a central unit (e.g., a pico base station) by a wire-based communications link, such as a T1 or E1 line. These high speed, high bandwidth lines typically do not include a separate clock line, but may nevertheless still be used to provide timing reference signals for phase locked loops within the radio heads. The phase locked loops are in turn used to perform a wide variety of tasks, such as carrier frequency synthesis. Typically, a network downlink signal is supplied to the central unit and the relevant timing reference signals are derived from the network downlink signal by an oscillator or other means within the central unit. However, there may be times when the network downlink signal is not available, but it may be desirable for the central unit to continue to provide the timing reference signal to the radio head(s), so as to allow the radio head(s) to continue operation and/or gracefully shut down on-going RF communications.
The present invention provides a method and apparatus for supplying a timing signal from a central unit to at least one radio head connected to the central unit via a communications link. The central unit transmits a downlink signal to the radio head(s) via the communications link based on a transmit clock supplied via a clock selector within the central unit. In a first (normal) mode, the clock selector supplies a clock signal derived from a network downlink signal received at the central unit as the transmit clock. However, in response to the loss of the network downlink signal, a second (secondary) mode is entered, wherein the clock selector instead supplies a clock signal derived from an uplink clock signal generated by the radio head, such as by a PLL therein, as the transmit clock. Thus, the central unit may supply a reference clock signal to the radio head even when the network downlink signal is temporarily lost.